Changes in accessibility of cellulose for kraft pulps measured by deuterium exchange

Aalto University, P.O. Box 11000, FI-00076 Aalto www.aalto.fi Author Raili Pönni Name of the doctoral dissertation Changes in accessibility of cellulose for kraft pulps measured by deuterium exchange Publisher School of Chemical Technology Unit Department of Forest Products Technology Series Aalto University publication series DOCTORAL DISSERTATIONS 68/2014 Field of research Forest Products Chemistry Manuscript submitted 14 February 2014 Date of the defence 13 June 2014 Permission to publish granted (date) 8 April 2014 Language English Monograph Article dissertation (summary + original articles) Abstract The emerging biorefinery processes and products set distinct processing and quality requirements compared to pulp and paper. Therefore, fundamental properties of cell wall, such as cellulose accessibility, require a more thorough understanding in the future.The aim of this study was to increase an understanding on the changes in cellulose accessibility throughout the chemical refining of wood, including kraft pulping, bleaching, drying, and further processing of the pulp. The characteristic physicochemical conditions in each subprocess were simulated with laboratory setups. In part, the analysis was based solely on literature. Cellulose accessibility was directly evaluated by deuteration, both in the liquid phase (deuteration combined with Fourier transform infrared spectroscopy) and in the vapor phase (dynamic vapor sorption with deuteration). The analytical methods based on deuterium exchange provided kinetic information on the underlying phenomena and revealed the reversible nature of cellulose microfibril coalescence. During kraft pulping, the proposed cellulose microfibril coalescence occurs mainly during the heat-up period and follows the first order dynamics. The active alkali (AA) content has a significant effect on the phenomenon. Hot acidic treatments, typical of ECF pulp bleaching, induce the formation of inaccessible regions in pulp similar to drying. Alkaline treatments, in contrast, induce a dynamic equilibrium state either between microfibrils and microfibril aggregates or between the amorphous and crystalline parts of cellulose. The equilibrium shifts towards disengagement at higher pH and temperature.The changes in cellulose accessibility in alkali are dependent on the drying history of the pulp. In addition, hemicellulose dissolution may affect the accessibility. Thus, TEMPO catalyzed oxidation of birch kraft pulp can be radically intensified by a short treatment with relatively mild alkali at room temperature. Multiple drying cycles or prolonged exposure to humid atmosphere increase cellulose microfibril coalescence, but this effect diminishes gradually. As a conclusion, many steps in processing wood and pulp reduce cellulose accessibility, presumably by microfibril coalescence. However, alkaline treatments of pulp induce an equilibrium state, which can increase the accessibility prior to further processing steps, e.g., catalytic oxidation of primary alcohols. Analytical methods applying deuteration provide valuable information on the changes in cellulose accessibility and their dynamics.The emerging biorefinery processes and products set distinct processing and quality requirements compared to pulp and paper. Therefore, fundamental properties of cell wall, such as cellulose accessibility, require a more thorough understanding in the future.The aim of this study was to increase an understanding on the changes in cellulose accessibility throughout the chemical refining of wood, including kraft pulping, bleaching, drying, and further processing of the pulp. The characteristic physicochemical conditions in each subprocess were simulated with laboratory setups. In part, the analysis was based solely on literature. Cellulose accessibility was directly evaluated by deuteration, both in the liquid phase (deuteration combined with Fourier transform infrared spectroscopy) and in the vapor phase (dynamic vapor sorption with deuteration). The analytical methods based on deuterium exchange provided kinetic information on the underlying phenomena and revealed the reversible nature of cellulose microfibril coalescence. During kraft pulping, the proposed cellulose microfibril coalescence occurs mainly during the heat-up period and follows the first order dynamics. The active alkali (AA) content has a significant effect on the phenomenon. Hot acidic treatments, typical of ECF pulp bleaching, induce the formation of inaccessible regions in pulp similar to drying. Alkaline treatments, in contrast, induce a dynamic equilibrium state either between microfibrils and microfibril aggregates or between the amorphous and crystalline parts of cellulose. The equilibrium shifts towards disengagement at higher pH and temperature.The changes in cellulose accessibility in alkali are dependent on the drying history of the pulp. In addition, hemicellulose dissolution may affect the accessibility. Thus, TEMPO catalyzed oxidation of birch kraft pulp can be radically intensified by a short treatment with relatively mild alkali at room temperature. Multiple drying cycles or prolonged exposure to humid atmosphere increase cellulose microfibril coalescence, but this effect diminishes gradually. As a conclusion, many steps in processing wood and pulp reduce cellulose accessibility, presumably by microfibril coalescence. However, alkaline treatments of pulp induce an equilibrium state, which can increase the accessibility prior to further processing steps, e.g., catalytic oxidation of primary alcohols. Analytical methods applying deuteration provide valuable information on the changes in cellulose accessibility and their dynamics.